Probability of Life‐Limiting Fatigue Failures in Titanium Alloys

Abstract

Fatigue lifetimes tend to separate into two populations as the stress level is decreased. The short lifetime population, termed as the life-limiting distribution, is found to be largely controlled by the small + long crack growth lifetimes. The longer lifetime population, which tends to dominate the mean-lifetime, has an increasing contribution of the crack-initiation lifetime as the stress level is decreased. Fatigue lifetime distribution in Ti-6Al-2Sn-4Zr-2Mo was studied from the perspective of the life-limiting versus the mean behavior. The crack-initiation mechanisms in the life-limiting versus the mean-lifetime failures were characterized using quantitative tilt fractography and electron back-scattered diffraction analysis, and the results were compared to other titanium alloys. A modeling approach for predicting the probability of occurrence of life-limiting failures is discussed. The approach takes into account the hierarchy of randomly occurring microstructural combinations, which is suggested to produce a distribution in fatigue deformation levels leading to a probability of a critical level of deformation accumulation for crack initiation in a very few cycles. The approach is illustrated with the help of a single phase -titanium microstructure for which predictions of the probability of occurrence of the life-limiting failures were made with respect to the stress level and the specimen volume.

Metallic and Ceramic Materials Research. Task Order 0005: Metallic, Materials, Methods, Characterization and Testing Research

“…In this study, the minimum lifetime mechanism or the minimum lifetime failure is referred to as the life-limiting mechanism or the life-limiting failure [4]. In addition to an accurate assessment of the minimum lifetime limit, it is also important to estimate the probability of occurrence of the life-limiting mechanism.…”